A cuttings re-injection (CRI) operation involves the collection and transportation of drilling waste (commonly referred to as cuttings) from solid control equipment on a rig to a slurrification unit. The slurrification unit subsequently grinds the cuttings (as needed) into small particles in the presence of a fluid to make a slurry. The slurry is then transferred to a slurry holding tank for conditioning. The conditioning process effects the rheology of the slurry, yielding a “conditioned slurry.” The conditioned slurry is pumped into a disposal well, through a casing annulus, into sub-surface fractures in the formation (commonly referred to as the disposal formation) under high pressure. The conditioned slurry is often injected intermittently in batches into the disposal formation. The batch process typically involves injecting roughly the same volumes of conditioned slurry and then waiting for a period of time (e.g., shutting-in time) after each injection. Each batch injection may last from a few hours to several days or even longer, depending upon the batch volume and the injection rate.
The batch processing (i.e., injecting conditioned slurry into the disposal formation and then waiting for a period of time after the injection) allows the fractures to close and dissipates, to a certain extent, the build-up of pressure in the disposal formation. However, the pressure in the disposal formation typically increases due to the presence of the injected solids (i.e., the solids present in the drill cuttings slurry), thereby promoting new fracture creation during subsequent batch injections. The new fractures are typically not aligned with the azimuths of previous existing fractures.
With large-scale CRI operations, release of waste into the environment must be avoided and waste containment must be assured to satisfy stringent governmental regulations. Important containment factors considered during the course of the operations include the following: the location of the injected waste and the mechanisms for storage; the capacity of an injection well or annulus; whether injection should continue in the current zone or in a different zone; whether another disposal well should be drilled; and the required operating parameters necessary for proper waste containment.
Modeling of CRI operations and prediction of disposed waste extent are required to address these containment factors and to ensure the safe and lawful containment of the disposed waste. Modeling and prediction of fracturing is also required to study CRI operation impact on future drilling, such as the required well spacing, formation pressure increase, etc. A thorough understanding of the storage mechanisms in CRI operations is a key for predicting the possible extent of the injected conditioned slurry and for predicting the disposal capacity of an injection well.
One method of determining the storage mechanism is to model the fracturing. Fracturing simulations typically use a deterministic approach. More specifically, for a given set of inputs, there is only one possible result from the fracturing simulation. For example, modeling the formation may provide information about whether a given batch injection will open an existing fracture created from previous injections or start a new fracture. Whether a new fracture is created from a given batch injection and the location/orientation of the new fracture depends on the alternations of local stresses, the initial in-situ stress condition, and the formation strength. One of the necessary conditions for creating a new fracture from a new batch injection is that the shut-in time between batches is long enough for the previous fractures to close. For example, for CRI into low permeability shale formations, single fracture is favored if the shut-in time between batches is short.
Once the required shut-in time for fracture closure is computed from the fracturing simulation, a subsequent batch injection may create a new fracture if the conditions favor creation of a new fracture over the reopening of an existing fracture. This situation can be determined from local stress and pore pressure changes from previous injections, and the formation characteristics. The location and orientation of the new fracture also depends on stress anisotropy. For example, if a strong stress anisotropy is present, then the fractures are closely spaced, however if no stress anisotropy exits, the fractures are widespread. How these fractures are spaced and the changes in shape and extent during the injection history can be the primary factor that determines the disposal capacity of a disposal well.